Demand for liquid crystal displays expands due to application for liquid crystal displays of liquid crystal televisions, personal computers, and the like. Usually, a liquid crystal display device is composed of a liquid crystal cell in which a transparent electrode, a liquid crystal layer, a light filter, and the like are sandwiched between glass plates and tow two polarizing plates provided at both sides of the liquid crystal cell. Each of the polarizing plates has a structure that a polarizer (also referred to as polarizing layer or polarizing film) is sandwiched between two polarizing plate protective films. Usually, as this polarizing plate protective film, a cellulose triacetate film is used.
On the other hand, technical progress in recent years accelerates a trend to enlarge the size of a liquid crystal display, and also diversifies the application of liquid crystal displays. For example, the application include utilization for large size displays installed in streets and shops and utilization for advertisement displays employing a display device called a digital signage installed in public places.
In such an application, since liquid crystal displays may be utilized at the outdoors, deterioration of polarizer due to high temperature quantity and high humidity becomes problems. Accordingly, polarizing plate protective films are required to have durability for high temperature quantity and high humidity. However, from conventional cellulose ester film such as cellulose triacetate film, it becomes difficult to obtain durability for high temperature quantity and high humidity.
On the other hand, polymethylmethacrylate (hereafter, abbreviated as PMMA) which is the representative of an acrylic resin exhibits excellent transparency and dimension stability as well as low moisture absorption, it is used suitably for a polarizing plate protective film.
However, the PMMA film is poor in heat resistance and causes problems of deformation during use under high temperature or during use for a long period of time. Further, as compared with cellulose ester film and the like, since acrylic film breaks easily and is brittle, handling of the acrylic film becomes difficult, and especially it is difficult to use the acrylic film to produce a polarizing plate protective film for use in a large size liquid crystal display device.
With reference to the above-mentioned problems, in order to improve moisture resistance and heat resistance, Patent document 1 proposes a technique to add polycarbonate (hereafter, abbreviated as PC) to an acrylic resin. However, since usable solvent is restricted, and compatibility between these resins is not sufficient, the mixture of these resins tends to cause cloudy. Therefore, it is difficult to use the mixture of these resins for a polarizing plate protective film.
Further, Patent document 2 proposes an optical film composed of a resin composite formed by blending an acrylic resin and a cellulosic.
The optical film made of this blended resin composite is a polarizing plate protective film which is good in transparency, has durability for high temperature and high humidity, and is excellent in brittleness. However, with the simple mixture of them, the resultant film is poor in saponification adaptability. Therefore, in order to paste the film onto a polarizer, it is necessary to use a special adhesive different from a polyvinyl alcohol type water-base adhesive used for a cellulose ester film being an ordinary polarizing plate protective film.